The molecular Biology Section has produced attenuated lines of human and rodent malarias and studied their developmental characteristics. We have defined auxotrophic mutants of both species of parasite as characterized by their dependency for growth upon either exogenously supplied pyrimidines or para-aminobenzoic acid. These should prove useful in the study of both the biochemistry of parasites and the development of immunity to parasites. We are also studying the complementation of yeast mutants with genes derived from malaria parasites in an effort designed to characterize genes involved in drug resistance, development, and sexual differentiation.